Automatic sweep circuit

ABSTRACT

An input signal is converted into a dc voltage proportional to the frequency of the input signal in a frequency-voltage converter, while saw tooth waves are generated in a saw tooth wave generator triggered with the input signal. The dc voltage is applied to a variable resistance circuit where the resistance is changed corresponding to the dc voltage. The period of such saw tooth waves is controlled with the variable resistance circuit so that the saw tooth waves may follow the input signal frequency for automatically indicating at all times a desired number of waveforms on the screen of a cathode-ray tube. The variable resistance circuit may be controlled with means for free running operation of the saw tooth wave generator during the absence of the input signal.

BACKGROUND OF THE INVENTION

This invention relates to a sweep circuit used in oscilloscopes and thelike, and more particularly to an automatic sweep circuit for indicatinga specific number of waveforms constantly on the screen of a cathode-raytube by automatically making a sweep frequency follow the frequency ofan input signal which may vary with time.

Generally, in prior-art sweep generators a sweep frequency suitable forthe frequency of any input signal must be selected by manually changinga sweep range as required. It is difficult to observe such waveformsmoving on the screen when the input signal frequency varies. There arevarious types of automatic sweep circuits now in use, which are howeverunsatisfactory in linearity of saw tooth waves or complicated in circuitdesign, and which are expensive if the linearity is improved.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of this invention to providean automatic sweep circuit which is relatively simple in circuitarrangement and which is inexpensive. It is another object of thisinvention to provide an automatic sweep circuit provided with means forfree running a saw tooth wave generator during the absence of an inputsignal applied thereto.

According to the invention a sweep frequency or the period of saw toothwaves is controlled with means having a field effect transistor theresistance of which is changed with a dc voltage applied thereto andproduced in proportion to the frequency of an input signal in afrequency-voltage converter, so that a specific number of waveforms canbe indicated constantly on the screen of a cathode-ray tube without anymanual adjustment.

The present invention will be more readily understood by reading thefollowing description in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing a preferred embodiment of anautomatic sweep circuit of the invention; and

FIG. 2 is a time chart explanatory of the circuit operation.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In a circuit shown in FIG. 1, a represents a one-shot multivibratorconsisting of resistances R₁ to R₇, capacitors C₁ to C₃, transistors Q₁and Q₂, and a diode D₁, which is commonly used for generating pulses Bof a specific width with an input signal A applied to a terminal (in). brepresents an integrator, consisting of resistances R₁₁ to R₁₃, acapacitor C₅, and an operational amplifier A₁, which produces a dcvoltage C proportional to the frequency of the input signal byintegrating pulses B. c represents a saw tooth wave generator,consisting of resistances R₁₄ to R₁₉, capacitors C₆ and C₇, transistorsQ₄ and Q₅, and a diode D₂, which generates saw tooth waves whentriggered with the input signal. The one-shot multivibrator a and theintegrator b constitute a frequency voltage converter FV. The integratorb may be formed of resistors and capacitors without using theoperational amplifier. In FIG. 1, d represents a variable resistancecircuit, consisting of a resistance R₂₀, a capacitor C₈, a field effecttransistor Q₆, and an operational amplifier A₂, where the dc voltage Cfrom the frequency voltage converter FV is applied to the minus inputterminal of the operational amplifier A₂ through the resistance R₂₀,amplifier A₂ having the plus input terminal thereof grounded. Thus, theminus input terminal may be kept constantly in imaginary short and atground potential within a control range of the operational amplifier A₂by controlling voltage at the gate of the transistor Q₆. The drain ofthe field effect transistor is connected to the minus input terminal,while the source is connected to the output of the saw tooth wavegenerator c. A reference letter e represents a circuit, consisting ofresistances R₂₁ and R₂₂, diodes D₃ and D₄, and a capacitor C₉, whichmakes the saw tooth wave generator c free running by supplying a bias tothe transistor Q₆ while no input signal A is fed to the terminal (in).In other words the circuit e makes the variable resistance circuit dinoperative for generation of saw tooth waves during the absence of theinput signal.

With such arrangement as described above, the operation of the inventionwill be explained. When an input signal (A) is supplied to the terminal(in), the one-shot multivibrator a triggered therewith may producepulses B of a fixed width, which are then integrated in the integrator band are converted into a dc voltage (C) that is proportional to thefrequency f₁ of the input signal A. On the other hand saw tooth waves Dare generated in the saw tooth wave generator c with the input signal(A) supplied thereto, and appear at a junction g of the capacitor C₈ forcharge and discharge and the source of the transistor Q₆. The dc voltage(C) from the frequency voltage converter FV is applied to the resistanceR₂₀ in the variable resistance circuit d, and a current flowstherethrough, which is given by the formula I₁ = E_(1v) / R₂₀, whereE_(1v) is the magnitude of the dc voltage. This current also flowsthrough the transistor Q₆, the resistance of which is thereforeequivalent to R_(Q).sbsb.6 = E_(2v) / I₁, where E_(2v) is the voltage atthe terminal g.

Now assuming an input frequency to be ten times f₁ or 10f₁, a current I₂is given by 10E_(1v) / R₂₀ = 10I₁. Hence the equivalent resistance ofthe transistor Q₆ is given by R'_(Q).sbsb.6 = E_(2v) / I₂ = E_(2v) /10I₁ = 1/10 × R_(Q).sbsb.6. This shows that an increase in the inputfrequency by ten times results in a decrease in the equivalentresistance of the transistor Q₆ by a factor of 1/10. Likewise, anincrease in the input frequency by 100 or 1000 times results in adecrease in the equivalent resistance of the same transistor by a factorof 1/100 or of 1/1000. Assuming a period of certain saw tooth waves tobe T₁, T₁ = (1/C₈.R_(Q).sbsb.6). Then, if the input frequency increasesten times, a period of such saw tooth waves is given by ##EQU1##

Similarly, if the input frequency increases 100 or 1000 times, a periodof such saw tooth waves may increase 100 or 1000 times. In this manner aspecific number of waveforms can be indicated at all times automaticallyon the screen of a cathode-ray tube.

The free running operation of the automatic sweep circuit as describedabove will now be explained. When the input signal (A) is not fed to theterminal (in), no dc voltage such as (C) is produced, and saw toothwaves cannot be generated as a result of an increase to almost infinityin the equivalent resistance of the transistor Q₆. To prevent this, thecircuit e consisting of diode D₃, resistance R₂₁, capacitor C₉,resistance R₂₂, and diode D₄ is provided in which a dc voltage isproduced by rectifying and smoothing the input signal through diode D₃,resistance R₂₁, and capacitor C₉. To this dc voltage is added a negativebias -B through the resistance R₂₂, so that positive potential may beproduced at a junction of the resistance R₂₁ and the capacitor C₉ duringthe presence of the input signal, while negative potential may beapplied to a junction of the drain of the transistor Q₆ and the minusinput of the operational amplifier A₂ through a diode D₄ during theabsence of the input signal at the terminal (in). Thus the circuit edoes not effect the transistor Q₆ while the input signal is present.When the input signal is absent, the negative bias -B is supplied to thetransistor Q₆ through the resistance R₂₂ and consequently the equivalentresistance R"_(Q).sbsb.6 of the field effect transistor is given by

    R" = E.sub.2v ÷ (-B/R.sub.20)

which is constant. Then saw tooth waves may be generated at a period ofT = (1/C₈.R"_(Q).sbsb.6) without any input signal being fed to theterminal (in). In this manner the variable resistance circuit d is madeinoperative for generation of saw tooth waves at the time of no inputsignal. The number of waveforms may be changed by varying the resistanceR₂₀.

As has been described heretofore, the invention permits automaticindication at all times of a specific number of waveforms of an inputsignal without manual adjustment of sweep ranges, highly stablesynchronization, and faster response when the input signal varies.Furthermore, linearity of sweep waves can be improved by feeding theoutput from the operational amplifier A₂ back to the gate of thetransistor Q₆. The invention makes it easy and simple to observefrequency responses and waveforms and to compare amplitudes of a varyinginput signal.

Although a particular embodiment of this invention has been illustratedand described, it will be understood that such is not intended to limitthe scope of the invention thereto, since many modifications may be madethereto. It is therefore contemplated by the appended claims to coverany such modifications as fall within the true spirit and scope of theinvention.

What is claimed is:
 1. An automatic sweep circuit comprising:frequencyvoltage converter circuit means connected to a source to receivetherefrom an input signal, to generate input pulses, and to convert saidpulses to a d.c. voltage proportional to the frequency of said inputsignal; saw tooth wave generator means connected to said source forgenerating saw tooth waves when triggered by said input signal; andvariable resistance circuit means, connected to said frequency voltageconverter means and to said saw tooth wave generator means, forcontrolling said saw tooth wave generator means by controlling theperiod of said saw tooth waves such that said saw tooth wavesautomatically follow said frequency of said input signal, said variableresistance circuit means comprising an operational amplifier having aninput connected to said frequency voltage converter circuit means toreceive therefrom said d.c. voltage and an output, and a field effecttransistor serially connected between the output of said saw tooth wavegenerator means and said input of said operational amplifier, said fieldeffect transistor having a gate connected to said output of saidoperational amplifier, the gate voltage of said field effect transistorthereby being controlled by said output of said operational amplifier.2. A circuit as claimed in claim 1, further comprising free-runningcircuit means, connected to said input of said operational amplifier,for rectifying said input signal and for applying a bias to said fieldeffect transistor and said operational amplifier only during the absenceof said input signal, and for thereby rendering said variable resistancecircuit means inoperative and said saw tooth wave generator meansoperative during the absence of said input signal.